/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Given an optional widget, try and get the most suitable parent window to use with dialogs (such as file and directory pickers). This will first try and get the window that owns the widget (if provided), before falling back to using the MainFrame window.
[TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > FindBestParentWindowForDialogs ( const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > & InWidget, const ESlateParentWindowSearchMethod InParentWindowSearchMethod )